Amperometric or fuel cell-type gas sensors typically include at least two electrocatalytic electrodes (an anode and a cathode), at least one of which is a gas diffusion electrode or working electrode. The working electrode can be either the anode or the cathode in any given sensor. The gas diffusion electrode typically includes fine particles of an electrocatalytic material adhered to one side of a porous or gas-permeable membrane. The gas sensor can also include a third, reference electrode to maintain the working electrode at a known voltage or potential.
The electrocatalytic side of the working electrode is in ionic contact with the second electrode (the counter electrode, whether the anode or the cathode) via an electrolyte (for example, a liquid electrolyte, a solid electrolyte or a quasi-solid state electrolyte). A liquid electrolyte is typically a solution of a strong electrolyte salt dissolved in a suitable solvent, such as water. Quasi-solid state electrolytes can, for example, include a liquid electrolyte immobilized by a high-surface-area, high-pore-volume solid. The working electrode and the counter electrode are also in electrical contact via an external circuit used to measure the current that flows through the sensor.
Various manufacturers of gas detectors include some means of monitoring the presence of an electrochemical gas sensor and determining its serviceability. One common method is to generate a suitable target gas (either the analyte of interest or a suitable stimulant) and monitor the response of the sensor to that generated gas. As typical gas generators are electrochemical cells themselves, there is a correlation between the amount of current used to produce the gas sample and the concentration of that sample. The method yields the presence of a working gas sensor and can be used to correct the output of the sensor. However, the technique has several disadvantages including, for example, complexity and ambiguity. In that regard, the gas generation cell is subject to the same forces of degradation to which the sensor is subject. Moreover, unless some method of monitoring the condition of the gas generator is employed, these methods can result in a self-consistent, but analytically incorrect indication of sensor health.
U.S. Pat. No. 6,370,940 describes a method for determining the concentration of a gas sample that could be used to actually calibrate the sensor if the concentration of the gas were known. The method requires a known concentration of test gas and the means to modulate the flow of the gas to the sensor.
In a number of current sensors, the presence of a sensor and sensor serviceability is determined via electronic testing. Calibration of such sensors requires measurement of sensor response during exposure to a standard calibration gas having a known concentration of analyte gas. For example, U.S. Pat. No. 6,428,684 discloses a method of determining the response of a sensor and comparing the determined sensor response with a “normal” response. The testing purportedly determines abnormalities in sensor operation and predicts future failure. In one embodiment, a potentiostat circuit is modified to allow the sensor to be tested galvanostatically. A small current flowing through the sensor for short time periods allows the electrode capacitance to be determined. Passing larger currents through the sensor, and especially by varying the current passed with time, provides a means to characterize the electrochemical properties of the sensor. Comparison of these electrical properties with reference values or with data obtained at a different time is used to determine the functional status of the sensor.
U.S. Pat. No. 6,049,283 describes a method of detecting the presence of a serviceable electrochemical gas sensor by measuring the electronic noise in the output of the sensor amplifier.
U.S. Pat. No. 6,629,444 describes a method of diagnosing defects in electrochemical gas sensors by suddenly changing the water vapor pressure of the air surrounding the sensor to more dry or more humid air thereby causing a sharp change in the acidity at the working electrode and hence a transient current in the sensor which can be used to monitor the sensor's condition.
U.S. Pat. No. 6,123,818 describes a method of detecting the presence of a serviceable electrochemical gas sensor by applying a transient to the non-inverting input of the operational amplifier that amplifies the output current of the sensor. The gain of that operational amplifier is monitored. If the gain resulting from the transient is high, a serviceable sensor is present; if the gain is low, a serviceable sensor is not present. U.S. Pat. No. 6,251,243 describes a similar method of detecting the presence of a serviceable gas sensor. Under this method, the transfer function of the operational amplifier is monitored.
U.S. Pat. No. 5,202,637 describes a method for detecting the presence of an electrochemical gas sensor by applying a potential pulse or a periodically varying potential to the sensor. The output current of the sensor is monitored. If a current is detected in response to the potential signal, then a sensor is present.
From this it is clear that it is desirable to develop improved devices, systems and methods for testing gas sensors and, preferably, devices, systems and methods suitable to correct the output of the gas sensor on the basis of an electronic test.